This invention relates to a process for producing a lithium cobalt oxide (LiCoO2) having a layered rock-salt type (xcex1-NaFeO2 type) structure. A powder of such lithium cobalt oxide is useful, e.g., as a cathode material for rechargeable lithium-ion batteries.
Due to their extraordinary energy density, rechargeable lithium-ion batteries are presently attracting attention as rechargeable power sources for use in portable electronic/electric devices such as portable telephones and notebook-type personal computers. Investigations are being made on the utilization of this type of batteries also as a large-capacity energy resource for, e.g., electric vehicles. Thus, the lithium-ion batteries are increasing in importance more and more.
The current rechargeable lithium-ion batteries mostly employ a lithium cobalt oxide (LiCoO2) having a layered rock-salt type (xcex1-NaFeO2 type) structure as a cathode material, a carbonaceous material such as graphite as an anode material, and a solution of any of various organic substances including Li salt as an electrolyte. In particular, the demand for LiCoO2 as a cathode material is expected to increase in the future. However, since this compound contains cobalt, which is a rare metal, it is one of the causes of the high costs of rechargeable lithium-ion batteries. Therefore, a means for obtaining the lithium cobalt oxide at lower cost is required.
The lithium cobalt oxide has conventionally been synthesized by firing a mixture of cobalt oxide with lithium carbonate in the air at 700 to 900xc2x0 C. For an attempt to reduce the high production cost due to high-temperature firing, it is necessary to find out a method in which the reaction is conducted at a lower temperature. However, a rechargeable lithium-ion battery employing a sample synthesized, e.g., at around 400xc2x0 C. as a cathode material has a discharge plateau around 3.5 V, which is lower than those of batteries employing a cathode material synthesized at 850xc2x0 C. (about 3.8 to 4 V). Hence, when a cathode material synthesized at such a low temperature is used as it is in a rechargeable lithium-ion battery, this leads to reduced battery performances (E. Rossen, J. N. Reimers and J. R. Dahn, Solid State Ionics, 62(1993)5; J. N. Reimeres and J. R. Dahn, J. Electrochem. Soc., 139(1992), 2091).
In view of the above, if a cathode material comparable in properties to a lithium cobalt oxide obtained through high-temperature firing can be produced through lower-temperature synthesis, this technique is extremely useful industrially.
Among low-temperature synthesis methods is hydrothermal synthesis. A prior art method for hydrothermal synthesis (air oxidation/hydrothermal method) is the use of CoOOH as a starting material to generate LiCoO2 at 160xc2x0 C. (D. Larcher, M. R. Palacin, G. C. Amatucchi and J. M. Tarascon, J. Electrochem. Soc., 144(1997), 408). However, since the cobalt in CoOOH, used in this method, is trivalent, it is usually necessary to oxidize beforehand an inexpensively available starting material of divalent cobalt (cobalt(II) chloride, cobalt(II) hydroxide, etc.). Namely, the above hydrothermal synthesis method necessitates two reaction processes respectively for CoOOH production and for reacting the obtained CoOOH with lithium.
Accordingly, the main object of this invention is to provide a technique for producing at a low temperature a layered rock-salt type LiCoO2, useful as a cathode material for rechargeable lithium batteries, from an inexpensive salt of divalent cobalt.
The inventor has made intensive studies in view of the above-described problems of prior art techniques. As a result, the inventor has succeeded in establishing a technique for producing a layered rock-salt type LiCoO2, useful as a cathode material for rechargeable lithium-ion batteries, from a salt of divalent cobalt by a hydrothermal synthesis method (hereinafter referred to as the hydrothermal oxidation method).
Specifically, the process of this invention for producing a lithium cobalt oxide (LiCoO2) having a layered rock-salt type structure by the hydrothermal oxidation method is characterized by hydrothermally treating at least one water-soluble cobalt salt in an aqueous solution containing a water-soluble lithium salt and an alkali metal hydroxide at 105 to 300xc2x0 C. in the presence of an oxidizing agent.
Examples of the water-soluble cobalt source for use in this invention include chlorides, nitrates, and sulfates of cobalt and hydrates of these and hydroxides of cobalt. Preferred of these cobalt sources are compounds of divalent cobalt (chloride, nitrate, sulfate, etc.). These cobalt sources may be used alone or in combination of two or more thereof.
The oxidizing agent may be, for example, water-soluble chlorate or peroxide of an alkali metal such as sodium or potassium. Specific examples thereof include NaClO3 and KClO3. Two or more oxidizing agents may be used in combination.
The water-soluble lithium source may be lithium chloride, nitrate, sulfate, hydroxide, etc. Specific examples thereof include lithium hydroxide (any of anhydrous and hydrated ones) and lithium chloride. These lithium sources may be used in combination of two or more thereof.
Examples of the alkali metal source include sodium hydroxide and potassium hydroxide (any of anhydrous and hydrated ones). These may be used in combination.
A general procedure of the process of this invention is described below. A water-soluble cobalt salt is dissolved in distilled water in an amount of usually about 0.01 to 1 mol/l (hereinafter abbreviated as M), preferably about 0.1 to 0.5 M, in terms of anhydride. A small amount (about 1 to 10 cc) of concentrated hydrochloric acid is added to the aqueous solution obtained. Thereto is added about 0.1 to 100 g (preferably about 0.2 to 5 g) of an oxidizing agent. Subsequently, about 1 to 100 g (preferably about 1.5 to 50 g) of a lithium compound is added to the mixture obtained. A solution of an alkali metal source compound such as sodium hydroxide or potassium hydroxide is further added in a concentration of about 1 to 100 M, preferably about 20 to 50 M to completely dissolve the oxidizing agent.
Subsequently, the mixture thus prepared is placed still in a hydrothermal reactor (e.g., autoclave) and subjected to a hydrothermal reaction. Although the hydrothermal reaction conditions are not particularly limited, the reaction is conducted usually at about 105 to 300xc2x0 C. for about 0.5 to 48 hours, preferably at about 200 to 250xc2x0 C. for about 1 to 24 hours.
After completion of the reaction, the reaction product is washed with distilled water and filtered in order to remove the starting materials remaining unreacted. The reaction product is then dried to obtain the desired layered rock-salt type LiCoO2. The above description should not be construed as limiting the scope of this invention, and the reaction can be conducted by other methods unless the essence of this invention is changed.
According to this invention, it has become possible to easily mass-produce a lathered rock-salt type LiCoO2, which has been difficult to industrially produce at low cost. Consequently, the development of rechargeable lithium-ion batteries containing LiCoO2 as the cathode material and the practical use thereof are more accelerated.